The present invention relates to electrochemical fuel cells. More particularly, the present invention relates to fluid flow field plates and to a method of fabricating such plates.
Electrochemical fuel cell assemblies convert reactants, namely fuel and oxidant, to generate electric power and reaction products. Electrochemical fuel cell assemblies generally employ an electrolyte disposed between two electrodes, namely a cathode and an anode. The electrodes generally each comprise a porous, electrically conductive sheet material and an electrocatalyst disposed at the interface between the electrolyte and the electrode layers to induce the desired electrochemical reactions. The location of the electrocatalyst generally defines the electrochemically active area.
Solid polymer fuel cell assemblies typically employ a membrane electrode assembly (xe2x80x9cMEAxe2x80x9d) consisting of a solid polymer electrolyte, or ion exchange membrane, disposed between two electrode layers. The membrane, in addition to being ion conductive (typically proton conductive) material, also acts as a barrier for isolating the reactant (that is, fuel and oxidant) streams from each other.
The MEA is typically interposed between two separator plates, which are substantially impermeable to the reactant fluid streams, to form a fuel cell assembly. The plates act as current collectors, provide support for the adjacent electrodes, and typically contain fluid flow field channels for supplying reactants to the MEA or for circulating coolant. The plates are typically referred to as fluid flow field plates. The fuel cell assembly is typically compressed to promote effective electrical contact between the plates and the electrodes, as well as effective sealing between fuel cell components. A plurality of fuel cell assemblies may be combined electrically, in series or in parallel, to form a fuel cell stack. In a fuel cell stack, a plate can be shared between two adjacent fuel cell assemblies, in which case the plate also separates the fluid streams of the two adjacent fuel cell assemblies. Such plates are commonly referred to as bipolar plates and may have flow channels formed therein for directing fuel and oxidant, or a reactant and coolant, on each major surface, respectively.
The fuel stream that is supplied to the anode typically comprises hydrogen. For example, the fuel stream may be a gas such as substantially pure hydrogen or a reformate stream containing hydrogen. Alternatively, a liquid fuel stream such as aqueous methanol may be employed.
The oxidant stream, which is supplied to the cathode, typically comprises oxygen, such as substantially pure oxygen or a dilute oxygen stream such as air.
The electrochemical reactions in a solid polymer fuel cell are generally exothermic. Accordingly, a coolant is typically also employed to control the temperature within a fuel cell assembly. Conventional fuel cells employ a liquid, such as, for example, water, to act as a coolant. In conventional fuel cells, the coolant stream is fluidly isolated from the reactant streams.
Fluid isolation is important for several reasons. For example, one reason for fluidly isolating the fuel and oxidant streams in a hydrogen-oxygen fuel cell is the reactivity of hydrogen and oxygen with each other. The MEA and separator plates are, therefore, substantially impermeable to hydrogen and oxygen. However, since the MEA also functions as an electrolyte, the membrane is generally permeable to protons and water (water is generally required for proton transport in membrane electrolytes).
Fluid flow field plates are generally formed from a suitable electrically conductive material. Furthermore, as indicated above, fluid flow field plates are typically made of a substantially fluid impermeable material (that is, a material sufficiently impervious to typical fuel cell reactants and coolants to fluidly isolate the fuel, oxidant and coolant fluid streams from each other). Expanded graphite, also known as flexible graphite, is a material that is employed in the manufacture of fluid flow field plates. Furthermore, because expanded graphite is a compressible material, an embossing process, such as roller embossing or reciprocal (also know as stamp) embossing, may be employed.
When compared with conventional engraving or milling methods, embossing processes typically provide cost and speed advantages. Embossing processes also typically minimize part handling, thereby making them better suited for mass production. Conventional embossing processes, more specifically reciprocal embossing and roller embossing, present different relative shortcomings with respect to one another.
Improved tolerances are typically achieved by reciprocal embossing. Indeed, the raw material fed into a reciprocal embossing apparatus (typically in the form of discrete sheets) can typically be thicker than the raw material fed into a roller embossing apparatus (typically in the form of rolled sheet material). The ability to work with a thicker piece of material generally results in improved accuracy, including achievement of dimensional tolerances and repeatability, in the mass fabrication of fluid flow field plates. In the production of elongated fluid flow field plates, however, which typically comprise substantially straight (that is, linearly extending), parallel channels extending between fluid distribution regions located at opposite ends of each fluid flow field plate (also referred to as header regions), the thickness of such an elongated plate as well as the parallelism of its channels over the plate""s entire length are more easily controlled by roller embossing.
On the other hand, reciprocal embossing is typically a slower and more expensive process than roller embossing. Indeed, reciprocal embossing typically involves higher tonnage equipment than roller embossing. Reciprocal embossing typically includes a pre-stamping requirement for removing air from the material to be embossed; such a pre-stamping requirement is typically not associated with roller embossing. Roller embossing also typically allows continuous processing of material to and/or from the equipment involved, and allows for continuous cleaning of embossing teeth; conversely, reciprocal embossing typically requires temporary interruptions of the processing of material to and/or from the equipment involved, both to allow for embossing and to allow for the cleaning of embossing teeth.
In light of the foregoing, roller embossing has at times been the preferred embossing process for the mass production of fluid flow field plates, especially the mass production of elongated fluid flow field plates. However, the roller embossing process does suffer from particular shortcomings.
Expanded graphite material with a suitable degree of flowability is typically used during a roller embossing process. As the expanded graphite material is extruded from the rolling dies, however, variations in plate length typically result. In order to account for the anticipated extrusion, difficult and expensive adjustments often need to be implemented. The expanded graphite material would typically be post-impregnated (that is, the material is impregnated after it has been embossed) with a curable polymeric composition or material (such as methacrylate), and then cured by conventional means (such as heating or radiation) in order to, among other things, improve the plates"" mechanical characteristics. A pre-impregnated material (that is, the material is impregnated before it has been embossed) is typically not employed because of the resultant increase in the number of variables that can detrimentally affect anticipated extrusion. Indeed, employing a pre-impregnated material, as opposed to a post-impregnated material, would typically increase the difficulty and expense of the adjustments needed to account for the anticipated extrusion.
For example, with respect to elongated fluid flow field plates that include substantially straight, parallel channels extending between fluid distribution regions located at opposite ends of each fluid flow field plate, the rolling dies include embossing teeth, which emboss the channels, and special inserts, which are incorporated into the rolling dies to form the fluid distribution regions. The position of the special inserts on the rolling dies need typically to be pre-corrected to account for the anticipated extrusion. As the anticipated extrusion may vary between batches of raw material fed into the roller embossing apparatus, the pre-correction step can result in a process that is difficult and expensive to implement, as the inserts may need to be removed and re-positioned a number of times on the surface of the rolling dies during production.
Another shortcoming of conventional roller embossing techniques is the introduction of residual stresses in certain areas of the fluid flow field plates, more specifically around transitional areas where the fluid flow field plates changes from one cross-sectional profile to another.
For example, with respect to elongated fluid flow field plates comprising substantially straight, parallel channels extending between fluid distribution regions located at opposite ends of each fluid flow field plate, residual stresses are typically introduced around areas of transition between the fluid flow field channels and the fluid distribution regions. These residual stresses often result in warping and/or breaking of the fluid flow field plates around such transitional regions, thereby leading to higher than desired rejections for a mass production process.
Accordingly, there is a general need for a method of mass production of fluid flow field plates that addresses the above-noted shortcomings. The present method overcomes such shortcomings, and provides further related advantages.
A method of fabricating a fluid flow field plate, which is suitable for use in an electrochemical fuel cell assembly, comprises (a) roller embossing a first embossed pattern in a sheet of compressible, electrically conductive material, and then (b) reciprocal embossing a second embossed pattern in the sheet.
The first embossed pattern can comprise a fluid flow field channel region. In one embodiment, the first embossed pattern comprises fluid flow field channels, more specifically substantially straight, parallel fluid flow field channels.
The second embossed pattern can comprise a fluid distribution region. In one embodiment, the second embossed pastern comprises manifold openings and supply channels.
The sheet material can be pre-impregnated with a curable polymeric composition, with the method further comprising a step of curing the pre-impregnated sheet material. Pursuant to the present method, the curing step is performed after the roller embossing and the reciprocal embossing steps. In one embodiment, the sheet material is pre-impregnated with expanded graphite.
The reciprocal embossing step can comprise simultaneously reciprocally embossing a first fluid distribution region of one fluid flow field plate and a second fluid distribution region of another fluid flow field plate. In one embodiment, the reciprocal embossing step comprises simultaneously reciprocally embossing a first set of manifold openings and supply channels of one fluid flow field plate and a second set of manifold openings and supply channels of another fluid flow field plate.
The reciprocal embossing step can further comprise cutting the sheet between the first fluid distribution region of the one fluid flow field plate and the second fluid distribution region of the other fluid flow field plate.
The reciprocal embossing step can be performed at preset length intervals of the roller embossed sheet material, the preset length equal to the desired length of the fluid flow field plate.
Specific details of certain embodiment(s) of the present method are set forth in the detailed description below to provide an understanding of such embodiment(s). Persons skilled in the technology involved here will understand, however, that the present method has additional embodiments, and/or may be practiced without at least some of the details set forth in the following description of preferred embodiment(s).